In alternating current arc welding installations, such as TIG welding operated in the A.C mode, the welding current alternates between positive and negative polarity; therefore, during each half cycle the welding current is zero at the zero crossing. At this instant, the arc between the electrode and workpiece tends to be unsustainable. Various techniques are employed for preventing erratic welding operation due to periodic extinguishing of the welding arc. In some power supplies, the current is instantaneously shifted between the polarities to provide insufficient time for the arc to be extinguished. However, a more common procedure is to provide high frequency, high voltage signals across the arc gap just after the zero crossing points of the welding current to strike and restrike the arc at the appropriate time. The present invention is directed toward a device and method of accomplishing this function. The most common arrangement for striking and restriking the welding arc is illustrated in U.S. Pat. No. 3,876,855, especially FIG. 1 and FIG. 3a. In the illustrated TIG welding process the arc is struck or restruck between the base metal, or workpiece, and the tungsten electrode with no physical contact by applying a high voltage, high frequency signal at an appropriate time during the welding cycle. The high frequency, high voltage generator used in the past, and illustrated in the above mentioned patent produces a high voltage, high frequency by a spark gap oscillating high frequency generator wherein a power supply is attached to a high frequency generating circuit through a transformer having a capacitor and inductance and a spark gap wherein the inductor and capacitor control the oscillation of the high frequency circuit. An inductor in this circuit is coupled between the input line of the power supply to the welding station by a coupling transformer to superimpose the spark created, high frequency signal on the welding current. The high frequency, high voltage created by the prior art spark gap device includes frequency components ranging over a wide band with the center frequency being in the range of 1.0-1.5 MHz. Consequently, there is substantial radiant energy interference generated by the arc starting device. Since the spark gap generators provide a large spectrum of transmitted signals, the energy across the arc gap of the welding installation is not well defined and substantial energy is applied at substantially lower frequencies than the base transmitted frequency of the spark generator. In addition stray higher frequencies can cause interference with associated logic within the welder itself and adjacent electronically controlled machinery and equipment. This is a real problem with use of standard TIG welders in factories where other digital control machinery exists. The spectrum of transmitted signals by the spark gap generator is controlled by the spark gap phenomenon and not by other parameters of the high frequency portion of the generator. A representative illustration of this basic concept employed for TIG welders is found in FIG. 3 of U.S. Pat. No. 3,876,855. This patent then discloses a circuit, as illustrated in FIG. 1 of the present application, to provide a kick voltage superimposed upon the welding current for the purpose of striking and restriking the arc plasma of a TIG welder when operated in an A.C. mode. This kick voltage will also function to start the arc in a D.C. mode of operation. In this type of prior art system, capacitor C is charged by a D.C. power supply E. In an appropriate instant, switch SW discharges capacitor C through the primary of coupling transformer T. This produces a kick voltage in the input line between terminals PS of the power supply and W of the electrode. The kick voltage concept of prior art, as illustrated schematically in FIG. 1 of the present application, produces a single pulse near the zero crossing of the welding current for a TIG welder. This starting arrangement has substantial disadvantages. A single high voltage pulse may not adequately start, strike or restrike the arc. Since there is no frequency involved, except for normal frequency distribution of the single pulse, the welding electrode must be protected against inadvertent contact with the operator at the time of the high voltage pulse. Further, the power supply E limits the output voltage of the single pulse for starting the plasma arc. Switch SW is a limiting factor of the magnitude of the output kick voltage. The voltage level is determined by the rate of discharge of the capacitor as well as the magnitude of the power supply E. To overcome some disadvantages of the prior art illustrated in FIG. 1 of the present application, U.S. Pat. No. 4,417,129 employs the prior art device illustrated in FIG. 2 of this application. In this device capacitor C is charged by a number of pulses through a FET switch periodically operated by pulses at transformer T1 under the control of programmed control circuit PC. After the capacitor C is charged, transformer T2 applies grid signal to vacuum tube VT for discharging the capacitor through coupling transformer T3. The capacitor C is charged incrementally to a value dependent upon the number of pulses to a primary of transformer T1. A pulse is fed to the primary of transformer T2, which pulse drives the high current capacity vacuum tube VT into conduction. When tube VT is fired, a high current pulse is fed through capacitor C1 to the primary of step up transformer T3. The pulse in the output of transformer T3 is in the range of 8-25 Kvolts dependent upon the number of pulses through switch FET. The pulse will have a minute width of about 1.0 ms by using a vacuum tube VT. Thus, a short, high voltage spike or kick voltage is created across the electrode and workpiece of the TIG welder at or near the zero crossing. This concept uses a single pulse, as in the prior art device illustrated in FIG. 1.
FIG. 3, which is not prior art, contains explanatory information and is presented for use in this background portion. If the kick voltage concepts of the prior art were modified to produce a series of pulses P1 at the zero crossing ZX of the welding current, pulses applied to the coupling transformer of a starting circuit would have a spacing defined by the commutation and resetting parameters of the kick voltage system. Thus, several pulses P1 could be created at the zero crossings to assure starting, striking and/or restriking of the plasma arc. However, the reset time of the circuit S would make the high voltage pulses P2 relatively widely spaced to produce high voltage at a relatively low frequency. This concept would not be advantageous for use in a standard TIG welder.
A single pulse kick voltage as disclosed in U.S. Pat. No. 3,876,855 is best employed in D.C. welding. In A.C. TIG welding a high frequency, high voltage signal, such as shown in FIG. 3a of this prior patent is normally employed for striking and restriking the arc.